1. Field of the Invention
The present invention relates to a steering characteristic controller for a power steering system wherein an electric current applied to an actuator which is provided in the power steering system for adjusting the steering characteristic is varied based upon the both of a output from a vehicle speed sensor and an output from a vehicle load sensor.
2. Discussion of the Background
In cargo vehicles such as station wagon, van, pickup, truck and the like, the weight of a load carried by the vehicle varies largely, and therefore, it is desirable to vary the characteristic of an assisting power generated by a power steering gear in adaptation not only for vehicle speeds but also for the weights of vehicle loads. To meet this requirement, there have been proposed power steering systems as disclosed in U.S. Pat. Nos. 4,212,366 and 4,216,841 both to T. Ohtuka et al.
In the known systems, as shown in FIG. 1, an electrically operable actuator 1 is provided for varying the characteristic of an assisting power generated by a power steering gear 2, and a control circuit 3 responsive to signals from a vehicle speed sensor 4 and a vehicle load sensor 5 is provided for controlling the operation of the actuator 1. The actuator 1 controls movement of an movable element (not shown) of a control valve 6 which is constructed to regulate the pressure of oil supplied to a reaction chamber formed in the steering gear 2 or to regulate the flow volume of oil supplied from an oil pump 7 to the power steering gear 2. The signals from the sensors 4, 5 are added by the control circuit 3 to control the operation of the actuator 1 based upon the added signal. Thus, a number of speed-assisting power characteristics are given to the system, and the steering gear 2 is enabled to generate the assisting power in accordance with one of the speed-assisting power characteristics which is automatically selected depending upon the weight of a load carried by the vehicle.
However, the electric current applied to the actuator 1 is in accordance with a characteristic shown in FIG. 2, wherein speed-electric current characteristics (S-Io) for different loads (W0, W1, W2, W3) are in parallel relation with one another. This causes the speed-assisting power characteristics given to the known systems to become in parallel relation with one another, whereby the variation rate of assisting power relative to vehicle speeds is maintained constant irrespective of vehicle loads. Accordingly, the known systems do not meet a further requirement for different variation rates between the speed-assisting power characteristics depending upon vehicle loads.